# 6. User Defined Type Conversion

In the process of creating Python bindings of a C++ library, most of the C++ classes will have wrappers representing them in Python land. But there may be other classes that are very simple and/or have a Python type as a direct counter part. (Example: a “Complex” class, that represents complex numbers, has a Python equivalent in the “complex” type.) Such classes, instead of getting a Python wrapper, normally have conversions rules, from Python to C++ and vice-versa.

>     // C++ class
>     struct Complex {
>         Complex(double real, double imag);
>         double real() const;
>         double imag() const;
>     };

>     // Converting from C++ to Python using the CPython API:
>     PyObject* pyCpxObj = PyComplex_FromDoubles(complex.real(), complex.imag());

>     // Converting from Python to C++:
>     double real = PyComplex_RealAsDouble(pyCpxObj);
>     double imag = PyComplex_ImagAsDouble(pyCpxObj);
>     Complex cpx(real, imag);

For the user defined conversion code to be inserted in the proper places, the “<conversion-rule>” tag must be used.

>     <primitive-type name="Complex" target-lang-api-name="PyComplex">
>       <include file-name="complex.h" location="global"/>

>       <conversion-rule>

>         <native-to-target>
>         return PyComplex_FromDoubles(%in.real(), %in.imag());
>         </native-to-target>

>         <target-to-native>
>           <!-- The 'check' attribute can be derived from the 'type' attribute,
>                it is defined here to test the CHECKTYPE type system variable. -->
>           <add-conversion type="PyComplex" check="%CHECKTYPE[Complex](%in)">
>           double real = PyComplex_RealAsDouble(%in);
>           double imag = PyComplex_ImagAsDouble(%in);
>           %out = %OUTTYPE(real, imag);
>           </add-conversion>
>         </target-to-native>

>       </conversion-rule>

>     </primitive-type>

The details will be given later, but the gist of it are the tags <native-to-target>, which has only one conversion from C++ to Python, and <target-to-native>, that may define the conversion of multiple Python types to C++’s “Complex” type.

Shiboken expects the code for <native-to-target>, to directly return the Python result of the conversion, and the added conversions inside the <target-to-native> must attribute the Python to C++ conversion result to the %out variable.

Expanding on the last example, if the binding developer want a Python 2-tuple of numbers to be accepted by wrapped C++ functions with “Complex” arguments, an <add-conversion> tag and a custom check must be added. Here’s how to do it:

>     <!-- Code injection at module level. -->
>     <inject-code class="native" position="beginning">
>     static bool Check2TupleOfNumbers(PyObject* pyIn) {
>         if (!PySequence_Check(pyIn) || !(PySequence_Size(pyIn) == 2))
>             return false;
>         Shiboken::AutoDecRef pyReal(PySequence_GetItem(pyIn, 0));
>         if (!SbkNumber_Check(pyReal))
>             return false;
>         Shiboken::AutoDecRef pyImag(PySequence_GetItem(pyIn, 1));
>         if (!SbkNumber_Check(pyImag))
>             return false;
>         return true;
>     }
>     </inject-code>

>     <primitive-type name="Complex" target-lang-api-name="PyComplex">
>       <include file-name="complex.h" location="global"/>

>       <conversion-rule>

>         <native-to-target>
>         return PyComplex_FromDoubles(%in.real(), %in.imag());
>         </native-to-target>

>         <target-to-native>

>           <add-conversion type="PyComplex">
>           double real = PyComplex_RealAsDouble(%in);
>           double imag = PyComplex_ImagAsDouble(%in);
>           %out = %OUTTYPE(real, imag);
>           </add-conversion>

>           <add-conversion type="PySequence" check="Check2TupleOfNumbers(%in)">
>           Shiboken::AutoDecRef pyReal(PySequence_GetItem(%in, 0));
>           Shiboken::AutoDecRef pyImag(PySequence_GetItem(%in, 1));
>           double real = %CONVERTTOCPP[double](pyReal);
>           double imag  = %CONVERTTOCPP[double](pyImag);
>           %out = %OUTTYPE(real, imag);
>           </add-conversion>

>         </target-to-native>

>       </conversion-rule>

>     </primitive-type>

## 6.1. Container Conversions

Converters for <container-type> are pretty much the same as for other type, except that they make use of the type system variables %INTYPE_# and %OUTTYPE_#. Shiboken combines the conversion code for containers with the conversion defined (or automatically generated) for the containees.

>     <container-type name="std::map" type="map">
>       <include file-name="map" location="global"/>

>       <conversion-rule>

>         <native-to-target>
>         PyObject* %out = PyDict_New();
>         %INTYPE::const_iterator it = %in.begin();
>         for (; it != %in.end(); ++it) {
>           %INTYPE_0 key = it->first;
>           %INTYPE_1 value = it->second;
>                   PyDict_SetItem(%out,
>                          %CONVERTTOPYTHON[%INTYPE_0](key),
>                  %CONVERTTOPYTHON[%INTYPE_1](value));
>         }
>         return %out;
>         </native-to-target>

>         <target-to-native>

>           <add-conversion type="PyDict">
>           PyObject* key;
>           PyObject* value;
>           Py_ssize_t pos = 0;
>           while (PyDict_Next(%in, &amp;pos, &amp;key, &amp;value)) {
>               %OUTTYPE_0 cppKey = %CONVERTTOCPP[%OUTTYPE_0](key);
>               %OUTTYPE_1 cppValue = %CONVERTTOCPP[%OUTTYPE_1](value);
>               %out.insert(%OUTTYPE::value_type(cppKey, cppValue));
>           }
>           </add-conversion>

>         </target-to-native>
>       </conversion-rule>
>     </container-type>

## 6.2. Variables & Functions

%in

> Variable replaced by the C++ input variable.

%out

> Variable replaced by the C++ output variable. Needed to convey the result of a Python to C++ conversion.

%INTYPE

> Used in Python to C++ conversions. It is replaced by the name of type for which the conversion is being defined. Don’t use the type’s name directly.

%INTYPE_#

> Replaced by the name of the #th type used in a container.

%OUTTYPE

> Used in Python to C++ conversions. It is replaced by the name of type for which the conversion is being defined. Don’t use the type’s name directly.

%OUTTYPE_#

> Replaced by the name of the #th type used in a container.

%CHECKTYPE[CPPTYPE]

> Replaced by a Shiboken type checking function for a Python variable. The C++ type is indicated by `CPPTYPE`.

## 6.3. Converting The Old Converters

If you use Shiboken for your bindings, and has defined some type conversions using the `Shiboken::Converter` template, then you must update your converters to the new scheme.

Previously your conversion rules were declared in one line, like this:

>     <primitive-type name="Complex" target-lang-api-name="PyComplex">
>       <include file-name="complex.h" location="global"/>
>       <conversion-rule file="complex_conversions.h"/>
>     </primitive-type>

And implemented in a separate C++ file, like this:

>     namespace Shiboken {
>     template<> struct Converter<Complex>
>     {
>         static inline bool checkType(PyObject* pyObj) {
>             return PyComplex_Check(pyObj);
>         }
>         static inline bool isConvertible(PyObject* pyObj) {
>             return PyComplex_Check(pyObj);
>         }
>         static inline PyObject* toPython(void* cppobj) {
>             return toPython(*reinterpret_cast<Complex*>(cppobj));
>         }
>         static inline PyObject* toPython(const Complex& cpx) {
>             return PyComplex_FromDoubles(cpx.real(), cpx.imag());
>         }
>         static inline Complex toCpp(PyObject* pyobj) {
>             double real =  PyComplex_RealAsDouble(pyobj);
>             double imag =  PyComplex_ImagAsDouble(pyobj);
>             return Complex(real, imag);
>         }
>     };
>     }

In this case, the parts of the implementation that will be used in the new conversion-rule are the ones in the two last method `static inline PyObject* toPython(const Complex& cpx)` and `static inline Complex toCpp(PyObject* pyobj)`. The `isConvertible` method is gone, and the `checkType` is now an attribute of the <add-conversion> tag. Refer back to the first example in this page and you will be able to correlate the above template with the new scheme of conversion rule definition.